This invention relates to the ink jet printing technology, and more particularly to an improved method of fabricating a plurality of printheads from two aligned and bonded substrates which are fastened together by a thermosetting adhesive and a UV curable adhesive inserted into alignment openings formed in portions of the substrates to hold the substrates together until the thermosetting adhesive is cured. The portions of the mated substrates having the alignment openings with the UV curable adhesive are discarded when the substrates are diced into a plurality of separate printheads.
Drop on demand jet printing systems can be divided into two basic types. One type uses a piezoelectric transducer to produce a pressure pulse that expels a droplet from a nozzle and, the other type uses thermal energy to produce a vapor bubble in an ink filled channel that expels a droplet. This latter type is referred to as thermal ink jet printing or bubble jet printing. Generally, thermal ink jet printing systems have a printhead comprising one or more ink filled channels that communicate with a relatively small ink supply chamber at one end, and have an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in the channels near the nozzle a predetermined distance upstream therefrom. The resistors are individually addressed with a current pulse representative of data signals, to momentarily vaporize the ink and form a bubble which expels an ink droplet.
One preferred method of fabricating thermal ink jet printheads is to form the heating elements on the surfaces of one silicon wafer and the channels and small ink supply chamber of reservoir in the surface of another silicon wafer. The two wafers are precisely aligned to insure that the heating elements are aligned to their corresponding channels, and then the two wafers are bonded together. The individual printheads are obtained by dicing the two bonded wafers. This general process has been described in Re. U.S. Pat. No. 32,572 to Hawkins et al. A critical part of this assembly process is the bonding adhesive and its application. Since two silicon wafers are mated that are extremely flat, a thin adhesive coating is sufficient to bond the two together, and a much thicker coat will clog the channels. U.S. Pat. No. 4,678,529 to Drake et al., describes a method of bonding the ink jet printhead components together by coating a flexible substrate with a relatively thin uniform layer of an adhesive having an intermediate non-tacky curing stage. About half of the adhesive layer is transferred from the flexible substrate to the high points or lands of one of the printhead components by placing it in contact therewith, and applying a predetermined temperature and pressure to the flexible substrate prior to peeling it from the printhead component. This causes the adhesive to fail cohesively in the liquid state, assuring that about half of the thickness of the adhesive layer stays with the flexible substrate and is discarded therewith, leaving a very thin uniform layer of adhesive on the printhead component lands. The transferred adhesive layer remaining on the printhead component enters an intermediate non-tacky curing stage to assist in subsequent alignment of the printhead components. The printhead components are aligned and the adhesive layer cured to complete the fabrication of the printhead.
U.S. Pat. No. 4,774,530 to Hawkins discloses an improved ink jet printhead which comprises an upper and lower substrate that are mated and bonded together with a thick film insulative layer sandwiched therebetween. The thick film layer is deposited on the substrate containing the heating elements and addressing electrodes and recesses are patterned in the thick film layer to expose the heating elements to the ink, thus placing them in a pit and to provide a flow path for the ink from the reservoir to the channels by enabling the ink to flow around the closed ends of the channels, thereby eliminating the fabrication steps required to open the channel grooves to the reservoir recess.
It has been found that movement of the aligned and mated wafers from an assembly fixture to a curing oven to cure the adhesive and permanently bond the wafers together frequently causes the wafers to become misaligned, therefore reducing the yield of printheads. The present invention eliminates the misalignment problem that occurs prior to complete curing of the bonding adhesive between the wafers.